Rotor blade



Dec. 4, 1962 w. CASTLES, JR

ROTOR BLADE Filed Jan. 11, 1957 INVENTOR.

WALTER O45715,J/Q.

United States Patent ()fifice 3,665,742 Patented Dec. 4, 1962 n1, it

3,066,742 RUTOR BLADE Walter Castles, Jr., Atlanta, Ga., assignor to Georgia Tech Research Institute, Atlanta, Ga., a corporation of Georgia Filed Jan. 11, N57, Ser. No. 633,655 1 Claim. (Cl. 176-459) This invention relates to rotor blades and is particularly concerned with the shape of air impeller blades such as may be appropriately used for high speed helicopters or converter-planes.

It is well recognized that the top speed of helicopters and in some instances that of converta-planes is limited by the occurrence of blade tip stall of the rotating blade. The tip-stall speed limitation arises from the necessity to balance out to zero the net rolling moment acting upon the rotor. As a consequence of this limitation, it is Well recognized that the top speed for helicopters and some converta-planes is comparatively quite low with respect to other forms of aircraft. This limitation is despite the fact that a great deal of excess power is available from the power plant.

It is therefore among the objects of the present invention to provide a novel and improved rotor blade for air impelling which will have an improved blade tip-stall characteristic.

More particularly, it is among the objects of the present invention to provide an improved impeller blade shape by which the blade tip-stall characteristics of the impeller blades of helicopters and/or converta-planes may be improved.

Another object of the invention is to provide a novel blade shape, the chord distribution of which is such as to materially increase the tip-stall limit top speed of a helicopter or converta-plane.

It is also an object of the present invention to provide a blade of the type defined in which the chord distribution is such as to provide an inverse taper for the outboard portion of the blade with respect to the inboard blade chord.

Other objects, features and advantages of the present invention will be apparent from consideration of the following specification taken in conjunction with accompanying drawings in which:

FIG. 1 is a top plan view of one form of blade carrying out the present inventive concept.

FIG. 2 is a side elevation of the blade of FIG. 1.

FIG. 3 is a modification of the blade embodying the inventive principle disclosed in FIG. 1.

FIG. 4 is a side elevation of the blade in FIG. 3.

FIG. 5 is a top plan view of further modification of the blade embodying my inventive concept.

FIG. 6 is a side elevation of the blade of FIG. 5.

While the blade shape of the present invention may be widely varied as indicated by the three forms disclosed in the drawings, it may be generally stated that the principle of the invention is concerned with the chord distribution of the blade in such manner as to provide inverse outward taper of the blade. Generally speaking, the blade of the present invention has a substantially constant inboard blade chord for over half of the blade length and preferably in the order of 0.7 of such length and thereafter the outboard portion of the blade is of greater Width. Preferably such extended width is by virtue of an inverse taper of the outboard portion and preferably following approximately the formula hereinafter set forth. It Will be understood in connection with the following specification that the invention is not concerned with the internal structure of the rotor blade or with the material thereof. The cross-sectional design may be presumed to follow the 2 normal airfoil section modified in accordance with the chord.

For the purpose of definition herein, the chord is defined as the distance from the leading edge of the blade airfoil to the trailing edge of the blade airfoil; the thickness as hereinafter used, means the maximum distance from the upper or one surface of the blade airfoil to the lower or other surface of the blade airfoil; the term inverse taper is a linear increase in airfoil chord with increase in the distance or radius from the hub at which that chord is measured, and the angularity is defined as the slope of the leading or trailing edge of the blade with re spect to the radial axis of the blade.

Referring now more particularly to the drawings, that form. of the invention shown in FIGS. 1 and 2 discloses the rotor blade conventionally mounted on the usual hub 10 through a connector 11 having a lever 12 for controlling the feathering of the blade by which its angularity is set. In this form of the invention the inboard portion of the blade is of uniform cross-section or chord as indicated at 13. At the outboard portion of the blade, the chord increases in a substantially uniform inverse taper as shown at 14. In this form of the invention the chord distribution is one in which the inboard chord c equals c and is essentially constant from the blade root to a radius which may be expressed as x R, where x is of the order of about 0.5 to about 0.9, the optimum being 0.7 and R is the radius of the blade tip. The inverse taper of the outboard portion 14 may be approximately described, using the letter 0 for the chord, by the following formula:

is from about 1.5 to about 3 x=nondimensional radius x =nondimensional r-adius at inboard end of inverse tip taper.

In other words, x is the actual distance from the hub to the chord c divided by the total distance from the hub to the blade tip and x is the distance from the hub to the beginning of the inverse taper divided by the total distance from the hub to the blade tip.

With such a blade it has been calculated that the fast Cessna CH-l helicopter which presently has a tip-stall limit top speed of 122 mph although at such speed only approximately 70 percent of the power available is utilized, may by using the formula given in the NACA TN 2656, by Walter Castles, Jr. and Noah C. New, July, 1952, published by National Advisory Committee for Aeronautics, have the following improved factors:

10 percent increase in the tip-stall limited top-speed where the rotor blades have the proportions x=0.70 and and;

15 percent tip-stall speed increase where the blades have the proportions a:=().70 and =3 in comparing the above it is to be noted that these ratios are for the same hovering mean blade lift coefficient; i.e. value of [263 where T Ct prrQFR 4 the rotor thrust coeflicient,

the nondimensional second moment of the blade area about the blade root, and b=nurnber of blades on rotor, consequently the present form does not entail any appreciable sacrifice in hovering performance. For a further consideration of the mathematical analysis of the present subject matter, reference may be had to the present inventors article as coauthor with Howard L. Durham in the October 1956 issue, vol. 1, No. 4 of the Tournal of the American Helicopter Society.

While the blade of FIGS. 1 and 2 is of uniform chord through the inboard portion 13 thereof, it will be seen that the invention contemplates an inboard taper, if such is desired, and thus in FIGS. 3 and 4 the inboard portion 23, comprising approximately 70 percent of the blade length is of an outwardly increasing taper and thus inversely tapered, as is the outboard terminal portion 24. But such taper is considerably more mild than that of the outboard portion 24; however, since the inverse taper of the inboard portion 23 is not critical no formula need be applied for this portion.

It will further be noted from FIGS. 5 and 6 that the enlargement of the outboard end 34 need not follow a uniform inverse taper. In this form of the blade the inboard portion 33 is of uniform chord for approximately 80 percent of the length of the blade while the outboard portion 34 is of a rounded or paddle like type of inverse enlargement.

As hereinbefore noted, the invention is not concerned with the material or internal structure of the blade; however, it is to be noted that in each instance the leading edge of the blade, here shown as the upper edge, is closer to the axis of the blade than the opposed trailing edge, and it is proposed that the axis of the blade be disposed approximately one quarter of the total chord of the base end of the bladefrom the leading edge.

In that form of the invention shown in FIGS. 3 and 4, the chord of the outboard portion Where it joins the tapered inboard portion 23 may be determined as approximately 1.5 times the chord at the base of the blade; in other words, the taper of the inboard portion from the base to the outboard portion is about one-half the length of the chord at the inboard portion of the blade while as suggested above the width of the extremity of the outboard portion of the blade is three times the width of the chord at the base.

While the transverse configuration of the blade may not be of critical value, it will be noted from the drawings in each instance the blade diminishes in thickness at the extreme end of the outboard portion, the bevel producing such diminishment being upwardly from the bottom with the top surface of the blade being substantially in the same plane throughout.

In considering the present invention it will, of course, be understood that the formulae here presented are suggestive and that in the practice of the invention various departures therefrom may be indicated in the light of further experimentation and in view of specific circumstances under which the impeller is to be used, thus it will be understood that numerous changes, modifications and the full use of equivalents may be resorted to in the practice of my invention without departing from the spirit or scope thereof as defined in the appended claim.

I claim:

A flexible air impeller blade particularly adapted for helicopters and the like, designed to increase the tipstall factor whereby speeds may be increased, having inboard and outboard portions and an airfoil section, the outboard portion having an inverse taper comprising approximately three tenths of the length of the blade and having a maximum chord c, approximately three times the chord of the base of the blade, chord c at any radius r of said inverse taper conforming substantially to the formula:

Where c =the blade chord at the base of the blade c =the blade tip chord R=the blade tip radius x=nondirnensional radius x =nondimensional radius at inboard end of inverse tip taper.

References Iited in the file of this patent UNITED STATES PATENTS 1,427,307 MaKenney Aug. 29, 1922 1,692,081 De La Cierva Nov. 20, 1928 1,855,660 Allen Apr. 26, 1932 2,014,242 Weichwald Sept. 10, 1935 2,070,657 Hefner Feb. 16, 1937 2,152,861 Bennett Apr. 4, 1939 2,532,371 Peterson Dec. 5, 1950 FOREIGN PATENTS 229,375 Germany Dec. 14, 1910 820,131 France Nov, 4, 1937 

